Atrial synchronous pacemakers are designed for use on patients whose hearts have normal atrial self-pacing, but, due to a defect in the conduction from the atrium to the ventricle, the ventricles fail to beat or keep pace with the atrial rhythm. Atrial synchronous pacemakers are designed to sense the naturally occurring atrial contractions of the heart, and to use them as a timing reference for generating electrical stimulation pulses to the ventricle of the heart. This is done by sensing the atrial contraction and providing a ventricular stimulation pulse after a short time delay which is selected to give atrial-ventricular synchrony. Usually a ventricular sensing and inhibit mode is also provided in the pacemaker, so that if a spontaneous ventricular contraction does take place within the A-V time interval following an atrial contraction, the ventricular contraction will be sensed and the pulse generating circuits inhibited and reset for that heartbeat cycle, since no ventricular stimulating pulse is required.
It is also common to provide backup (minimum rate) ventricular pacing in an atrial synchronous pacemaker, so that the heart will be maintained at the minimum rate in the event that the natural atrial contraction rate drops too low as in the case of atrial bradycardia. Also, in the case of a malfunction of the atrial lead or atrial sensing amplifier, it is important that the pacemaker continue to pace in the ventricular backup mode.
A problem sometimes occurs in prior art atrial synchronous pacemakers when operating in the minimum rate ventricular backup maode, in that under certain circumstances it is possible that the pacemaker may deliver a ventricular stimulating pulse too rapidly after a previous ventricular contraction. This too-close coupling of a stimulation pulse may fall in the vulnerable period of the ventricles during repolarization from the previous ventricular contraction, and may lead to dangerous consequences, including ventricular fibrillation. Also, under those circumstances the electrocardiogram might easily be misunderstood by medical personnel as a malfunctioning pacemaker.
The cause of this problem is the dissociation of atrial and ventricular activity in the heart during the ventricular backup mode, and the subsequent sensing of an atrial contraction (P-wave) occurring very closely after a ventricular event. This may occur in the case of atrial bradycardia wherein the natural atrial rate drops to a rate which is below the minimum ventricular backup rate, which, for example, might be 60 beats per minute. Under those circumstances the ventricle of the heart is paced at the 60 beat per minute ventricular backup rate, but assuming no retrograde conduction in the heart, the atrium continues to self-pace at its lower rate. With the atria and ventricles now operating independently and at different frequencies, the P-wave of the electrocardiogram is no longer fixed or synchronized with the QRS-wave complex. On successive pulses, the P-wave will drift into and through the QRS complex. Eventually a P-wave will occur just as the atrial sensing amplifier is turned on at the end of its refractory period following a ventricular event, or very shortly thereafter. The atrial sensing amplifier will then detect the P-wave and cause delivery of a ventricular stimulation pulse. The pulse may be delivered one A-V delay period later or may be constrained by a rate limit circuit. The result in either case is that the ventricle is supplied with a rapid ventricular stimulation pulse after too short a time interval following the previous ventricular contraction, and this rapid stimulation pulse may fall in the vulnerable period.
It is not practical to solve the above problem by further lengthening the refractory period of the atrial sensing circuit. Although to do so would safely move the quickest sensed P-wave to beyond the vulnerable period following a ventricular depolarization, such a scheme would effectively lower the maximum atrial tracking rate limit of the pacemaker. This is a poor result because many patients with the type of problem requiring an atrial synchronous pacemaker may still exercise normally and reach heart rates in excess of 150 beats per minute. However, with a lengthened atrial refractory period, the atrial sensing circuits would not be ready to receive every P-wave at these rapid normal heart rates.
It is possible to avoid the above-noted problem of too closely coupled pulses through the use of a dual sense/dual pace atrial-ventricular pacemaker. That type of device is not subject to the same problem, because the atrium is paced to maintain the minimum rate, which keeps both the atria and ventricles synchronized. Therefore, dissociation of the P and R-waves does not occur. While a dual sense/dual pace pacemaker solves this problem, it may use considerably more current than an atrial synchronous pacemaker, and therefore is subject to the disadvantage of shortened battery life or larger size. A need therefore remains for atrial synchronous pacemakers, for those patients in which full dual sense/dual pace atrial-ventricular synchronous pacing is not needed.
The present invention solves the above problem by providing a variable upper rate limit, and providing sensing means for switching to the upper, normally high limit (which is programmable and typically may be 150 beats per minute) during atrial synchronous pacing, and for automatically switching to a lower upper rate limit (for example 100 beats per minute) when operating in the backup ventricular pacing mode. When atrial synchrony is regained, the upper rate limit is automatically returned to the higher programmed value. In this manner the atrial sensing circuit is effectively prevented from sensing a P-wave immediately following an R-wave, while at the same time atrial synchronous operation at high heart rates during exercise is not precluded.